Bicycle rear suspension

ABSTRACT

A bicycle comprises a frame ( 10 ) a rear wheel ( 14 ) and a rear suspension, which comprises a swing arm ( 17 ) carrying the wheel and two pivot links ( 18, 19 ) spaced from one another in the fore and aft direction of the bicycle and coupling the arm to the frame to form, together with the frame, a four-bar linkage movable between a first end settings in an unloaded state of the suspension and a second end setting in a loaded state of the suspension. The links ( 18, 19 ) are each pivotably connected with the frame outwardly of the wheel circumference and are arranged so that on movement of the linkage from the first to the second end setting an instantaneous pivot centre ( 21 ) defined by the intersection of two axes ( 22, 23 ) each containing the pivot points of a respective one of the links ( 18, 19 ) moves downwardly and rearwardly with respect to the fore and aft direction.

[0001] The present invention relates to a bicycle and has particularreference to a rear wheel suspension in a bicycle.

[0002] Bicycles intended for off-road and other rough terrain usescommonly have rear suspension systems permitting sprung movement of therear wheel relative to the bicycle frame. A basic system is representedby a rearwardly extending swing arm carrying the rear wheel andpivotably connected to a seat tube or down tube of the bicycle frame. Aspring and damper unit is coupled between the spring arm and the frameto control pivot movement of the arm under suspension travel. The swingarm is usually a sturdy cast or fabricated member which is suitablystiff in bending and which can incorporate robustly formed journals forpivot connection to the frame and the spring and damper unit. Thesturdiness of the swing arm is highly desirable for acceptance of theshock loads acting on the suspension during off-road use, but the simplearc executed by the swing arm during suspension travel imposes acompromise with respect to the suspension behaviour. In particular, thedrive force transmitted to the rear wheel by the usualchain-and-sprocket transmission during a power stroke applied by a ridertends to pivot up the swing arm and wheel independently of bump responseof the suspension. The arm is then able to pivot down again in theinterval until the next power stroke, which results in a bobbing effector at least some degree of feedback from the foot pedal cranks whichusually feed the drive force to the transmission.

[0003] These disadvantageous effects can be counteracted by more complexmulti-element suspension linkages, of which the four-bar linkage isparticularly effective from the viewpoint of controlling rear wheelmovement in the course of suspension travel. Such a linkage can,depending on the position of its pivot centre, generate acounterbalancing force tending to pull the rear wheel down in oppositionto the tendency of the wheel to move up during the power stroke of therider. Generation of the counterbalancing force requires a relativelyhigh pivot centre of the linkage and such a high location also confersthe advantage that the rear wheel path has a more rearward initialdirection to enable a better response to bumps. However, a high pivotcentre location introduces the problem of disturbance to the pedalaction by a constantly changing chain length, thus pedal feedback, whenthe suspension has to cope with a continuously bumpy surface. Inaddition, large changes in chain length, i.e. changes in effectivelength caused by increase and decrease in the length of an idle zone ofthe chain, cannot be readily absorbed by conventional gear changingsystems based on chain displacement between coaxial sprockets ofdifferent diameter.

[0004] Problems of this nature can be resolved by designing the four-barlinkage so that its instantaneous pivot centre moves down and backtowards the pedal crank axis as the linkage displaces under progressive,bump-induced suspension travel. One such linkage is disclosed in U.S.Pat. No. 5,509,679 and subsequent continuation specifications, in whichthe linkage is composed of two rearwardly extending lower arms disposedone on each side of the wheel and pivotably connected to a frame seattube, two upwardly extending parallel upper arms similarly disposed oneon each side of the wheel and pivotably connected to the lower arms nearthe wheel axis and a short upper link pivotably coupling the upper armsto the seat tube at a spacing above the point of connection of the lowerarms. The length of seat tube between the two pivot connectionsrepresents the fourth element of the linkage. A spring and damper unitis angled between the short upper link and the frame. The relationshipand dimensions of the constituent elements of this four-bar linkage havethe result that the instantaneous pivot centre—represented by theinstantaneous point of intersection of the two notional axesrespectively containing the fulcra of the short upper link and thefulcra of the lower arms—does indeed displace rearwardly and downwardlyduring linkage compression. However, the length of the lower arms andthe point of pivot connection to the frame impose a limit on the extentof vertical shift of the instantaneous pivot centre. More significantly,the provision of paired upper and lower arms, the lengths of these armsand the pivot interconnection thereof near the rear wheel axis detractfrom the rigidity of this part of the linkage and the suspension as awhole tends to be appreciably less robust and less resistant to lateralflexing than suspension systems based on a single swing arm. The sitingof the spring and damper unit also imposes constraints and, in practice,modification of the frame to a more complex shape has been needed toaccommodate a spring and damper unit offering an effective stroke.

[0005] Other permutations of four-bar linkage rear suspensions incommercially available bicycles include a design with a triangular wheelcarrier frame representing one element of the linkage and articulated tothe bicycle frame, part of which forms another element of the linkage,by a short upper link and a short lower link below the upper link. Theupper link has an extension providing a coupling point for a spring anddamper unit connected to the bicycle frame down tube. This design yieldsprimarily a downward displacement of the instantaneous pivot centreduring suspension bump response, but without significant approach to thepedal crank axis, and the triangular carrier frame is a comparativelybulky component of the linkage. Moreover, lateral stiffness isprejudiced by the vertical relationship of the two links, whichco-operate to define a hinge zone. A further known design overcomes thebulkiness of the carrier frame by provision of a robust swing arm whichis articulated to the bicycle frame by a short upper link directedupwardly from the swing arm and a short lower link arranged below theupper link and directed forwardly and downwardly from the arm. Althoughthe swing arm itself possesses structural rigidity, resistance tolateral flexing is again compromised by the disposition of the two linksapproximately in vertical alignment and the instantaneous pivot centreof the linkage moves forwardly rather than rearwardly during suspensioncompression, thus producing displacement of the pivot centre in adirection actually opposite to that desired to counteract bobbing.

[0006] The invention therefore has as its principal objective theprovision of a bicycle with a rear suspension system which achieves thedesired degree of suspension compliance in conjunction with resistanceto bobbing, but without a penalty in terms of structural rigidity,especially resistance to lateral flexing.

[0007] A supplementary object is the provision of a rear suspensionlinkage which can be readily accommodated in a conventional bicycleframe design, thus without obliging special shaping, and for which adegree of freedom exists in specific dimensioning and disposition of thelinkage elements. A further supplementary object is the design of asuspension with commonality of some parts, so as to ease productioncosts. Yet another supplementary object is the creation of a suspensionlayout with scope for variable mounting of springing and damping meansto enable variation of springing and damping rates by simple measures.

[0008] Other objects and advantages of the invention will be apparentfrom the following description.

[0009] According to the present invention there is provided a bicyclecomprising a frame, a rear wheel and a rear suspension which comprises aswing arm carrying the rear wheel and two pivot links spaced from oneanother in the fore and aft direction of the bicycle and coupling thearm to the frame to form therewith a four-bar linkage movable between afirst end setting in an unloaded state of the suspension and a secondend setting in a loaded state of the suspension, wherein the links areeach pivotably connected with the arm and the frame outwardly of thewheel circumference and are arranged so that on movement of the linkagefrom the first to the second end setting an instantaneous pivot centredefined by the intersection of two axes each containing the pivot pointsof a respective one of the links moves downwardly and rearwardly withrespect to said direction at a first rate over an initial part of thetravel of the pivot centre and at a second rate reduced in relation tothe first rate over a subsequent part of the travel of the pivot centre.

[0010] Through use of a swing arm and disposition of the pivotconnections of the linkage outwardly of the wheel circumference thesuspension can be based on a wheel carrier member meeting allrequirements of strength, rigidity and relative ease of manufacture. Theswing arm can be a cast, forged or fabricated component of desiredshape, for example cranked or bent in its length to provide clearancefor other components, such as a main sprocket wheel of achain-and-sprocket drive of the bicycle. The two links can be kept shortso that they can be constructed to be particularly stiff in bending andthus maintain overall rigidity of the four-bar linkage consisting of therigid swing arm, the normally inherently stiff frame and the links. Therigidity of the linkage is enhanced by the spacing of the links in thefore and aft direction of the bicycle so that the linkage does notcontain a hinge zone formed by a vertical or more-or-less vertical planecontaining the pivot points of two of the links. At the same time,however, these links are disposed so that on upward movement of the rearwheel under bump response of the suspension the instantaneous pivotcentre of the linkage moves downwardly and rearwardly with respect tothe fore and aft direction. This provides the desired pivot behaviour ofthe four-bar linkage with respect to control of the direction of wheelmovement to counteract the bobbing effect occurring, as described in theintroduction, by tensioning of the chain of the chain-and-sprocketdrive. In addition, because the movement of the instantaneous pivotcentre over an initial part of its travel takes place at a first rateand over a subsequent part of its travel at a second rate reduced inrelation to the first rate it is possible for the pivot centre toinitially move relatively quickly when upward pivotation of thesuspension occurs and thereafter to move at a slower rate.

[0011] The link orientations can be selected so that a rearward one ofthe links pivots through a greater angle than a forward one of the linksduring an initial phase of movement of the linkage from the first to thesecond end setting and through generally the same angle as the forwardlink during a final phase of that movement. Consequently, the initialmovement of the pivot centre can be substantially along the axis of theforward link with only slight change in the position of that axis. Forpreference, the locus of the instantaneous pivot centre during itsmovement describes a first arc which is substantially convex upwardlyand a second arc which is substantially concave upwardly. This locusessentially results from an initial upward and subsequent downwardpivotation of the forward link. During the movement of the pivot centre,the locus thereof can be such as to pass through the pivot point of theforward one of the two links at the connection thereof with the frame,in particular at the point of transition from the upward to downwardmovement of that link or from the convex to the concave arc. Thedirection of movement of the pivot centre can be generally towards theaxis of a pedal crank drive of the bicycle and the centre is preferablydisposed adjacent to the axis of that drive in the second end setting ofthe linkage. This has the effect that the path of the rear wheel axisduring compression of the suspension initially gives rapid increase inthe distance between the axes of the rear wheel and the pedal crankdrive, thus growth in length of a chain coupling a drive sprocket anddriven sprocket respectively associated with the pedal crank drive andthe rear wheel, and subsequently virtually no growth.

[0012] The orientations of the two links are preferably selected to besuch that the rearward link extends forwardly and the forward linkrearwardly from the arm with respect to the fore and aft direction ofthe bicycle, the forward link preferably also extending downwardly fromthe arm. The angular disposition of the links when they extend forwardlyand rearwardly in the manner described is for preference such that theyextend convergently in direction away from the rear wheel in the firstend setting of the linkage. A particularly compact disposition of thelinks satisfying the requirements for their relative positioning isachieved if the rearward link is connected to the frame at a seat tubethereof and the forward link to the frame at a down tube thereof.

[0013] With respect to construction, the links can be substantiallyidentical in shape and size so as to achieve a significant costadvantage in production as well as savings in procurement andstock-holding. The connection of the links to the swing arm and theframe can be effected by connecting means comprising, for example,deep-groove full complement bearings which provide substantiallyplay-free pivot location of the links. The links can be machined ormoulded components so that, for example, bearing fit can be accuratelycontrolled without need for subsequent welding or heat treatment.Similarly, the frame and swing arm need only be drilled to accommodatepivot pins for the bearings. For preference a single continuous pivotpin is provided at each pivot point, which further contributes to thelateral stiffness of the suspension particularly at the locations mostsusceptible to play, i.e. the coupling points of the elements of thefour-bar linkage. Other forms of connecting means are possible,including location of pins in the links and bearings in the arms andframe.

[0014] The suspension can be completed by a spring and damper unitpivotably connected with the arm and frame to provide sprung and dampedtravel of the linkage between its end settings. Location between the armand frame allows scope for an effective stroke of the unit withoutobliging special shaping of members of the frame to the disadvantage ofstructural simplicity and strength and/or aesthetics. In addition toeasier accommodation of the unit, provision can be made for adjustmentof the position of the unit relative to the arm or frame for variationin the springing and damping rates. Due to the feature of rearwardmovement of the instantaneous pivot centre of the four-bar linkage undermovement of the suspension, the initially large spacing of the wheelaxis from that centre, which yields a large wheel to spring/damperratio, subsequently reduces to correspondingly reduce the ratio.Consequently, by adjustment of the inclination of the line of action ofthe spring and damper unit relative to the axis of the forward one ofthe links, which changes the perpendicular distance of that line ofaction from the pivot centre, the wheel to spring/damper ratio can bealtered in accordance with specific requirements. The adjustmentfacility can be such as to be readily carried out by the rider.

[0015] An embodiment of the present invention will now be moreparticularly described by way of example with reference to theaccompanying drawings, in which:

[0016]FIG. 1 is a schematic elevation of the rear part of a bicycleembodying the invention; and

[0017]FIGS. 2a-e are schematic diagrams showing five progressive stagesof movement of a rear suspension linkage in the bicycle part illustratedin FIG. 1.

[0018] Referring now to the drawings there is shown part of a bicycle,for example a bicycle intended for off-road use (“mountain” bicycle),comprising a frame 10 with a crossbar 11, down tube 12 and seat tube 13.The crossbar 11 and down tube 12 may be united into a single componentto the right of FIG. 1 and, whether so united or remaining separate, areconnected to a steering assembly and front wheel forks or suspension(not shown) of the bicycle. At the junction of the down tube 12 and seattube 13 there is provided a bearing location for a pedal crank drivefixedly connected with a drive sprocket of a chain-and-sprocket drivetransmission (not shown) to a rear wheel 14 of the bicycle. A drivensprocket of the transmission is fixedly connected with an axle of therear wheel and drive is transmitted from the drive sprocket to thedriven sprocket by way of an endless chain. In the case of a drivetransmission provided with gearing, several coaxial drive or drivensprockets of respectively different diameter may be provided togetherwith a chain displacing device (derailleur mechanism) for displacing thechain between the sprockets of different diameter. Pedal crank drivesand chain-and-sprocket drive transmissions are conventional componentsof bicycles and accordingly are not illustrated in FIG. 1. However, theaxis of the pedal crank drive and drive sprocket(s) is denoted by 15 andthe axis of the rear wheel axle and the drive sprocket(s) by 16.

[0019] The rear wheel 14 is suspended by a rear suspension comprising aswing arm 17 which carries an axle rotatably mounting the rear wheel, anupper, rearward link 18 extending forwardly of the arm and pivotablyconnecting the swing arm with the seat tube 13 and a lower, forward link19 extending rearwardly of the arm and pivotably connecting the swingarm with the down tube 12. The swing arm, links and the part of theframe between the pivot connection points of the links represent theelements of a four-bar linkage. The swing arm 17 is fabricated fromsheet aluminium or aluminium alloy, but could equally well be producedfrom other materials and could be cast, forged, machined or constructedin any other suitable manner. It is cranked in its length to rise abovethe zone of the drive sprocket of the transmission and to provide ashape compatible with the desired disposition of the coupling points ofthe four-bar linkage elements. The two links 18 and 19 are shortcomponents machined from aluminium, aluminium alloy, steel or othermaterial or made by forging, casting, stamping, fabricating, moulding orany other suitable method from metallic or other appropriate material ormaterials, preferably a material of light weight and high strength. Eachlink has two accurately machined bores providing locations fordeep-groove, full complement bearings, i.e. bearings with bearing ballsin direct contact with one another, rather than separated by webs of acage, and seated in deep grooves providing both radial and lateral(thrust) location of the balls. The bearings receive pivot pins fixed inthe swing arm and in the frame tubes or brackets attached thereto. Thetwo links can be identical in construction to provide savings inmanufacturing and purchasing cost.

[0020] The four-bar linkage constructed and arranged as described in thepreceding paragraph represents a particularly sturdy wheel-carryingstructure with good resistance to lateral flexing. This resistance ispromoted by the short lengths of the links 18 and 19 and theirdisposition at a spacing in the fore and aft direction of the bicycle;this spacing ensures that there is no coincidence, in a vertical ornearly vertical plane, of pivot points of the links such as to form ahinge zone reducing the transverse stiffness of the suspension.

[0021] The bicycle additionally includes a spring and damper unit 20which is pivotably connected with the swing arm 17 in the vicinity ofthe bend therein and with a bracket between the crossbar 11 and downtube 12. The connection of the unit 20 with the arm 17 can be adjustableto enable variation in the orientation of the unit in a vertical planerelative to the bicycle fore and aft direction.

[0022] As is evident from FIG. 1, the swing arm 17, under the control ofthe remaining elements of the four-bar linkage, is able to pivotupwardly in response to bumps encountered by the rear wheel 14 duringuse of the bicycle. In particular, the linkage is able to move between afirst end setting in an unloaded state of the bicycle, thus with therear wheel in a lowermost position, and a second end setting in a fullyloaded state of the bicycle, thus with the rear wheel in an uppermostposition. The end settings are defined by, respectively, maximumextension and maximum compression of the spring and damper unit 20. Thelengths and relative dispositions and orientations of the links 18 and19 are such that during travel of the linkage from the first end settingto the second end setting the rearward link 18 pivots upwardly (at itsrearward end) in the course of an initial phase of such travel while theforward link 19 pivots upwardly (at its forward end) only very slightly.In the course of a further and final phase of such travel the rearwardlink 18 continues to pivot upwardly while the forward link 19 now pivotsslightly downwardly. The forward link thus executes a reciprocatingmotion. This action of the links 18 and 19 produces a shift in theinstantaneous pivot centre of the linkage and consequently a desireddisplacement of the swing arm 17 and rear wheel 14 from the viewpoint ofcounteracting the bobbing effect induced in the chain-and-sprockettransmission by the power stroke exerted by the rider of the bicycle.The instantaneous pivot centre, which is denoted by 21 in FIG. 1, isdefined by the point of intersection of an axis 22 containing the pivotpoints of the rearward link 18 and an axis 23 containing the pivotpoints of the forward link 19.

[0023] The locus 24 of the instantaneous pivot centre 21 under upwardpivotation of the swing arm 17 is evident from the five diagrams ofFIGS. 2a to 2 e, which illustrate different positions of the swing armin the course of such movement and thus different settings of thefour-bar linkage between its first setting (FIG. 2a) and second endsetting (FIG. 2e). As shown, the locus 24 describes—progressingrearwardly from the foremost disposition of the pivot centre 21—ashallow convex arc and then a shallow concave arc. The locus passesthrough the point of pivot connection of the lower link 19 with theframe, at which point the transition from the convex portion to theconcave portion of the locus occurs. The relatively small deviation ofthe locus from the axis 23 of the forward link 19 reflects the minimalpivotation of that link, which manifests itself during the initial phaseof movement of the linkage from the first to the second end setting as arelatively rapid rearward and downward travel of the instantaneous pivotcentre 21. Only in the final phase of travel of this pivot centre do thetwo links 18 and 19 move at similar rates through a similar angle, whichultimately produces a relatively slow movement of the centre and bringsit into close proximity with the axis 15 of the pedal crank drive. Asexplained in the introduction, this course of movement of the pivotcentre represents the desired course from the viewpoint of minimisingchanges in chain length so as to counteract bobbing or pedal feedback.In addition, the minimal change in chain length assists the action of achain-displacing or derailleur mechanism if provided.

[0024] The described pivot action of the four-bar linkage has an equallyadvantageous influence on the springing and damping supplied by thespring and damper unit 20. When the linkage is in its first end setting,i.e. the suspension unloaded (FIG. 2a), the distance between the rearwheel axis 16 and the instantaneous pivot centre 21 is large and thewheel to spring/damper ratio is similarly large. In the second endsetting of the linkage (FIG. 2e), the distance is significantly smallerand the wheel to spring/damper ratio is smaller. If the unit 20 has anappropriately adjustable point of pivot connection with the swing arm 17or frame 10, the inclination of the unit relative to the axis 23 of theforward link 19 can be altered. This effectively changes theperpendicular distance of the line of action of the unit 20 from thepivot centre 21 so as to enable possible selection of a rising, linearor falling wheel to spring/damper ratio; the particular configurationillustrated in the drawings provides a slightly rising rate.

[0025] A bicycle embodying the invention, thus a bicycle incorporating arear wheel suspension as described in the foregoing, combines a sturdywheel-carrying structure with an intelligent wheel path under suspensionaction and additionally offers the possibility of an intelligent wheelto spring/damper ratio.

1. A bicycle comprising a frame, a rear wheel and a rear suspensionwhich comprises a swing arm carrying the rear wheel and two pivot linksspaced from one another in the fore and aft direction of the bicycle andcoupling the arm to the frame to form therewith a four-bar linkagemovable between a first end setting in an unloaded state of thesuspension and a second end setting in a loaded state of the suspension,wherein the links are each pivotably connected with the arm and theframe outwardly of the wheel circumference and are arranged so that onmovement of the linkage from the first to the second end setting aninstantaneous pivot centre defined by the intersection of two axes eachcontaining the pivot points of a respective one of the links movesdownwardly and rearwardly with respect to said direction at a first rateover an initial part of the travel of the pivot centre and at a secondrate reduced in relation to the first rate over a subsequent part of thetravel of the pivot centre.
 2. A bicycle as claimed in claim 1, whereina rearward one of the links pivots through a greater angle than aforward one of the links during an initial phase of movement of thelinkage from the first to the second end setting and throughsubstantially the same angle as the forward link during a final phase ofthat movement.
 3. A bicycle as claimed in claim 1 or claim 2, whereinthe locus of the instantaneous pivot centre during said movement thereofdescribes a first arc which substantially convex upwardly and a secondarc which is substantially concave upwardly.
 4. A bicycle as claimed inany one of the preceding claims, wherein the locus of the instantaneouspivot centre during said movement thereof passes through the pivot pointof a forward one of the links at the connection thereof with the frame.5. A bicycle as claimed in any one of the preceding claims, wherein thedirection of said movement of the instantaneous pivot centre isgenerally towards the axis of a pedal crank drive of the bicycle.
 6. Abicycle as claimed in claim 5, wherein the instantaneous pivot centre isdisposed adjacent to the axis of the drive in the second end setting ofthe linkage.
 7. A bicycle as claimed in any one of the preceding claims,wherein a rearward one of the links extends forwardly from the arm withrespect to the fore and aft direction.
 8. A bicycle as claimed in anyone of the preceding claims, wherein a forward one of the links extendsrearwardly from the arm with respect to the fore and aft direction.
 9. Abicycle as claimed in claim 8, wherein the forward one of the linksadditionally extends downwardly from the arm.
 10. A bicycle as claimedin any one of the preceding claims, wherein a rearward one of the linksextends forwardly and a forward one of the links rearwardly from the armand the links extend convergently in direction away from the rear wheelin the first end setting of the linkage.
 11. A bicycle as claimed in anyone of the preceding claims, wherein a rearward one of the links isconnected with the frame at a seat tube thereof.
 12. A bicycle asclaimed in any one of the preceding claims, wherein a forward one of thelinks is connected with the frame at a down tube thereof.
 13. A bicycleas claimed in any one of the preceding claims, wherein the links aresubstantially identical.
 14. A bicycle as claimed in any one of thepreceding claims, wherein the links are connected with the swing arm andthe frame by connecting means comprising deep-groove, full complementbearings.
 15. A bicycle as claimed in any one of the preceding claims,wherein the links are machined or moulded components.
 16. A bicycle asclaimed in any one of the preceding claims, wherein the links areconnected with the swing arm and the frame by connecting meanscomprising a single continuous pivot pin at each pivot point.
 17. Abicycle as claimed in any one of the preceding claims, wherein a springand damper unit is pivotably connected with the arm and frame to providesprung and damped travel of the linkage between its end settings.
 18. Abicycle as claimed in claim 17, wherein the spring and damper unit isadjustable in position relative to the arm and frame to vary thespringing and damping rates of the unit.